The present invention relates generally to heat transfer systems, and more particularly to the removal of low-boiling constituents from organic heat transfer fluids.
In the operation of many industries, heat transfer systems are employed to provide a heat source for many processes which are operated in the industrial plants. Oftentimes, organic heat transfer fluids are chosen to act as the heat transfer medium. In those instances, the organic heat transfer fluids are heated to the desired temperatures, usually between 400.degree. F. and 750.degree. F., and then pumped through piping or the like to one or more sites where the hot organic heat transfer fluids heat one or more processes.
Since the volume of fluid in the system varies with temperature, the systems contain an expansion tank or storage tank wherein a pad of inert gas, such as nitrogen, is usually maintained in a specified range of pressure over the hot organic heat transfer fluid. If the volume of fluid decreases, inert gas is added. In the event the pressure exceeds the maximum limit, such as when the volume of fluid increases, a control valve or manual bleed allows gas to escape.
It is known that organic heat transfer fluids degrade at high temperatures to form low-boiling constituents such as paraffinic or aromatic components or mixtures thereof. The higher the temperature, the faster the fluids degrade. These low-boiling constituents enter the inert gas pad in the expansion tank and, consequently, are released to the atmosphere when the pressure control valve activates. Fresh fluid is added to replenish the fluid lost due to degradation. Since fluids initially added to the heat transfer system may degrade at elevated temperatures and these systems typically house 2,000 to 4,000 gallons of fluid, a great deal of hydrocarbons are currently being vented to the atmosphere. A solution to this problem is one object of the present invention.
Water may also be present in the fluids. Water is undesirable in heat transfer systems because the presence of water and organic acids, which are formed from the oxidation of the fluids, can lead to the corrosion of metal components of the heat transfer systems. Moreover, water raises the vapor pressure of the system, which may cause vapor lock and relief valves to open which loses product. While water can be removed by pressure venting described above, this method is highly inefficient and does not achieve sufficiently low levels of water. Nevertheless, this method is often used in the start-up of a heat transfer system despite that it may take weeks to remove the water to begin actually running the system at elevated temperatures. A solution to this problem is another object of the present invention.
The operation of heat transfer systems as described above has been practiced virtually unchanged for decades.